Here is my explanation:
A router is a device that makes communication between different networks possible.
On these days mostly used on IP-Networks it can be just a program or OS (like NT/W2K/XP) or a piece of Hardware.
A Router is a gateway too at the same time - because he is the gateway between different networks.

This is a very instant explanation...if you need more info's I will try to tell more...

hubs and switches operate on layer2 of the osi model (the ethernet layer).

All ports on a hub are in the same collision domain, i.e. the ethernet frames from all the hosts connected can collide...

Switches create separate collision domains on each port by creating a forwarding table of the mac addresses of the hosts connected to each port. This way they only forward ethernet frames out the ports they are intended to rather than everyone.

Routers operate on layer3 of the osi model (think IP addresses) and are used to route ip packets between different 'networks' or 'broadcast domains'.

All hubs and most dumb switches exist -within- these broadcast domains or networks, while the routers act as 'gateways' between them. So, a gateway is just another name for the router that will route traffic off of your local LAN segment.

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what are the differences between a router and a switch with an uplink port

if I understand correctly, a network is "everything inside the walls of a castle" and a gateway is any means of going beyond the wall
if this is true, 2 gateways are required for inter-network communication (one on each side) ... ?

the switch doesn't know about ip addresses. it works on a lower layer of the stack.

Think of it this way...zip codes and street addresses.

Routers are like post offices(although many manage traffic for more than 1 zip code). The post office sends letters to zip codes. to route mail, no one looks at street addresses, they look at zip codes.

Once it gets to the destination post office, it dumps off to the carrieres in the zip code to deliver to a street address.
ip=zip code
mac address(ethernet)=street address

so, the switch will be connected to the router, but only to send traffic that doesn't belong on that network. The actual decision is handled at the ip layer though, but that's the principle.

And yes, your analagy is correct with the castles and gateways. Routers provide gateways to other networks. The return traffic has the same requirements.

Routers segment broadcast domains. Routers look at layer 3 the network layer. They base switching, and path determination on IP addresses. Switches operate at layer 2 or datalink layer. They segment collision domains. They base their switching on a MAC address. The MAC address is a hexidecimal number burned into your NIC card. It is always the same number. The Ip address is assigned to you by your ISP, it can change from day to day. Going along with what pedrow said with post offices. you can think of your MAC as your name, no matter where you are your name is always the same. However, if you move tomorrow your address might change, like your IP does.
To send information to anyone on your local network you must have their IP and their MAC, so your computer sends a broadcast to find which ever one it does not know. This broadcast will go through switches and hubs. However, it will not go through a router, the router will stop the broadcast or change the broadcast before sending it on to other networks. This is why a router segments broadcast domains. A switch stops the physical signal from being passed to every computer on the network based on MAC address. It knows which computer has each MAC address so it only goes to that computer. That is why it segments collision domains. A hub forwards everything through. It operates on layer 1, the physical layer. All a hub does typically is reshapes the data so it can travel further. I hope this helps, between this and what pedrow said you should be pretty clear.

That router could be at the phone company CO or at your location, depending on your particular install, but basically the gateway is the ip address of the 'way out' of the local LAN. Usually it's a router.

BTW, hubs do not operate at layer2 as suggested in the answer you accpted. Next time you accept answers double check what all the experts are telling you before you decide. If there is a disagrement ask for clairification.

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